1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to an intermediate image transfer type of color image forming apparatus.
2. Description of the Background Art
Generally, an intermediate image transfer type of image forming apparatus includes an image carrier, an intermediate image transfer body, primary image transferring means for transferring a toner image from the image carrier to the intermediate image transfer body, and secondary image transferring means for transferring the toner image from the intermediate image transfer body to a sheet or similar recording medium. This type of image forming apparatus is disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2002-214932. The image carrier, configured to carry a toner image corresponding to image data, is implemented as a photoconductive drum by way of example. For the intermediate image transfer body, use is often made of an endless, intermediate image transfer belt passed over a plurality of rollers. To effect primary image transfer, an electric field is formed between the drum and the belt. For secondary image transfer, an electric field and/or pressure is applied between the belt and a sheet.
Japanese Patent Laid-Open Publication No. 2000-010415, for example, teaches an intermediate image transfer type of color copier, color laser beam printer or similar color image forming apparatus. The apparatus taught in this document sequentially transfers toner images of different colors to an intermediate image transfer belt one above the other and then transfers the resulting composite color image to a sheet.
A problem with the intermediate image transfer type of image forming apparatus is that when image formation is repeated, image transferability is lowered or image transfer becomes irregular due to aging, as determined by experiments. One cause of the above problem is that resistivity on the surface of the belt to which a bias is applied varies due to repeated image formation. A change in the surface resistivity of the belt directly translates into a change in adequate bias and other image transfer conditions, lowering transferability or, when they locally vary, rendering image transfer irregular. More specifically, when the surface resistivity of the belt decreases due to aging, a current easily flows on the surface of the belt to which the bias is applied. If the amount of current is large, then a current expected to contribute to image transfer decreases with the result that transferability is lowered or toner scattering occurs due to an increase in electric field in a non-image transfer region.
In a tandem image forming apparatus including a plurality of image carriers, the distance between nearby primary image transferring means is small. Consequently, if the surface resistivity of the surface applied with the bias is low, then a current easily flows on the surface of the belt and causes, if large in amount, nearby primary image transferring means to interfere with each other.
In an image forming apparatus configured to apply a secondary image transfer bias to the inner or reverse surface of the belt, a current is apt to flow along the inner surface of the belt. This also causes the problems discussed above to arise.
Another cause of low transferability and irregular image transfer ascribable to aging is that the volume resistivity of the belt decreases as image formation is repeated. This also causes the image transfer conditions to vary as when the surface resistivity of the belt varies, bringing about the problems stated above.
It is known that the variation of resistance stated above occurs because the belt is subject to electric adverse influence, i.e., so-called hazard ascribable to, e.g., repeated bias application. To protect the belt from such deterioration ascribable to aging, Japanese Patent Laid-Open Publication Nos. 08-054789 and 09-281814, for example, propose to detect information dependent on the resistance of the belt and control a bias for image transferring means by taking account of the information detected.
The bias control scheme, however, cannot obviate irregular image transfer because the resistance of the belt does not uniformly vary due to the influence of toner and sheet. Further, a current flows along the surface of the belt due to the fall of resistance, so that interference between nearby image transferring means cannot be obviated. Moreover, in the case where a voltage used for primary image transfer is susceptible to the area of a toner image or the thickness of a toner layer, transferability varies between a single-color image and a composite color image with the result that image transfer is apt to become short or excessive.
In the intermediate image transfer type of image forming apparatus, irregular image transfer sometimes occurs on the surface of the belt in the event of primary and secondary image transfer, as also determined by experiments. One cause of this irregularity is that an irregular potential distribution, which is the replica of the potential of a latent image formed on the drum, sometimes appears on the belt at the time of primary image transfer. If the belt with such an irregular potential distribution enters a primary image transfer nip, then irregular image transfer occurs in accordance with the above irregular potential distribution.
More specifically, when a latent image is formed on the drum, a surface potential difference occurs between the image portion and the non-image portion or background of the drum. The surface potential difference remains on the drum even when the latent image is developed. When the drum faces a primary image transfer roller or similar primary image transferring means via the belt at the primary image transfer nip, a potential difference occurs between the image portion and the non-image portion relative to the roller. An electric field for primary image transfer is strong in the portion where the potential difference is great or weak in the other portion where it is small. A great amount of current flows in the portion where the electric field is strong, so that the surface potential of the belt becomes higher in the above portion than in the portion where the electric field is weak. If such an irregular potential distribution remains up to the next primary image transfer nip, then primary image transfer efficiency varies and brings about irregular image transfer.
Another cause of irregular image transfer is that the potential of the belt becomes irregular due to charge deposited on the belt in the event of secondary image transfer. Such irregularity is ascribable to the fact that the surface potential of the belt, remaining after the belt has moved away from the secondary image transfer nip, is different from the portion of the belt facing a sheet to the portion not facing it.
Why the potential difference occurs on the surface of the belt moved away from the secondary image transfer position will be described hereinafter. A current flows more easily in the non-facing region of the belt not facing a sheet than in the facing region of the same facing the sheet. As a result, when the secondary image transfer bias is applied from a secondary image transfer roller or similar secondary image transfer member, more current flows in the non-facing region than in the facing region. Consequently, more charge is fed to the non-facing region than to the facing region raising the surface potential of the non-facing region. It follows that the surface potential of the belt moved away from the secondary image transfer position is higher in the non-facing region than in the facing region. If such an irregular potential distribution remains on the belt up to the primary image transfer position following the secondary image transfer position, then a difference in primary image transfer efficiency occurs in accordance with the potential difference of the belt, resulting in irregular image transfer corresponding to the irregular potential distribution. Therefore, if the next toner image is transferred to the belt over the portions different in potential from each other, density becomes irregular accordingly.
Laid-Open Publication No. 2002-214932 mentioned earlier proposes to obviate irregular density ascribable to the potential contrast of the belt by reducing the contrast when the facing region and non-facing region of the belt pass the secondary image transfer nip. More specifically, for such a purpose, the above document switches the current value of the secondary image transfer bias between the time when the belt faces a sheet and the time when it does not face the sheet. Although this scheme can switch secondary bias control between the facing region and non-facing region of the belt, it cannot do so when the facing region and non-facing region exist together in the widthwise direction of the belt. It follows that irregular image transfer cannot be obviated when, e.g., a sheet of size A4 is passed in a landscape or a profile position or when a sheet of size B5 or similar relatively small size is passed.